The present invention generally relates to compound semiconductor devices and more particularly to an improvement of a group III-V compound optical semiconductor device for use in optical telecommunication, such as a laser diode or a photodiode. Further, the present invention relates also to an improvement of a high-speed group III-V compound semiconductor device.
Today, a telecommunication trunk generally uses an optical telecommunication system in which optical fibers are used for carrying information traffic in the form of optical signals. Currently, quartz glass optical fibers having an optical transmission band of 1.3 .mu.m or 1.5 .mu.m wavelength are used commonly. In correspondence to the foregoing specific transmission band of the optical fibers, current optical telecommunication systems generally use a GaInAsP double-heterojunction laser diode that includes an active layer of In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y and a cladding layer of InP. In such a GaInAsP double-heterojunction laser diode, the carriers are accumulated in the active layer by a potential barrier formed in the conduction band and the valence band between the GaInAsP active layer and the InP cladding layer, and stimulated emission of photons is substantially facilitated in the active layer by the carriers thus accumulated therein. In order to obtain a laser oscillation at the wavelength that matches the optical transmission band of the quartz glass optical fibers, the compositional parameter x for Ga and the compositional parameter y for As are adjusted appropriately in the foregoing GaInAsP laser diode.
However, such a conventional laser diode that uses a double-heterojunction structure of GaInAsP and InP for the accumulation of carriers has suffered from the problem of relatively large threshold current of laser oscillation and poor temperature characteristic, primarily due to the relatively small band discontinuity (.DELTA.Ec) of the conduction band between the GaInAsP active layer and the InP cladding layer. More specifically, the electrons escape easily from the active layer in such an GaInAsP laser diode because of the small potential barrier .DELTA.Ec formed by the foregoing band discontinuity, and a large drive current has to be supplied in order to sustain a laser oscillation in the active layer. This problem becomes particularly acute at high temperatures in which the carriers experience an increased degree of thermal excitation. Further, the foregoing GaInAsP laser diode has a problem in that the laser oscillation wavelength tends to shift to a longer wavelength side at high temperatures due to the temperature dependence of the bandgap of GaInAsP. It should be noted that the bandgap of GaInAsP decreases with temperature. This shift of the laser oscillation wavelength raises a serious problem particularly in a wavelength multiplex transmission process of optical signals.
In order to avoid the foregoing problems, conventional GaInAsP double-heterojunction laser diodes for use in optical telecommunication trunk or submarine optical cable systems have used a temperature regulation device, such as a Peltier cooling device, such that the operational temperature of the laser diode is maintained at a predetermined temperature.
On the other hand, there is a strong impetus to expand the use of optical telecommunication technology not only in the telecommunication trunks but also to subscriber systems. In relation to this, there is a demand for optical semiconductor devices suitable for use in home terminals.
When realizing such optical home terminals, it is essential that the optical home terminal is compact and low cost. Further, the optical home terminal should consume little electric power. In order to meet such demands, it is necessary to provide a laser diode that is operable in the 1.3 or 1.5 .mu.m band with a low threshold current, but without a temperature regulation.
As long as the foregoing GaInAsP/InP double-heterojunction system is used, the foregoing demand cannot be satisfied. Thus, efforts are being made to construct a laser diode having an active layer of GaInAs on a GaAs substrate such that a large band discontinuity .DELTA.Ec is secured in the conduction band. By increasing the In content in the GaInAs active layer, it is possible to reduce the bandgap energy Eg of the active layer, and the oscillation wavelength of the laser diode approaches the desired 1.3 .mu.m band. However, such an increase of the oscillation wavelength by increasing the In content in the GaInAs active layer is successful only to the point in which the oscillation wavelength reaches about 1.1 .mu.m. Beyond that, the lattice misfit between the GaInAs active layer and the GaAs substrate becomes excessive and the epitaxial growth of the GaInAs active layer is no longer possible on the GaAs substrate. It should be noted that the foregoing limit of 1.1 .mu.m takes into consideration the contribution of strain that acts in the direction to increase the oscillation wavelength of the laser diode.
In view of the foregoing situation, Japanese Laid-Open Patent Publication 7-193327 proposes a laser diode operable in the 1.3 or 1.5 .mu.m band, in which an active layer of GaInAs is sandwiched by a pair of cladding layers having a composition set such that a large band discontinuity .DELTA.Ec is secured between the active layer and the cladding layer and that the cladding layer has simultaneously a lattice constant close to that of a strained buffer layer provided on a GaAs substrate with a composition of Ga.sub.0.8 In.sub.0.2 As. However, the proposed device is deemed to be unrealistic in view of the large lattice misfit between the active layer and the GaAs substrate. It is believed that the existence of such a large lattice misfit reduces the lifetime of the laser diode substantially.
On the other hand, Japanese Laid-Open Patent Publication 6-37355 describes a compound semiconductor structure that includes a GaInNAs mixed crystal film formed on a GaAs substrate. By adding N to GaInAs, it becomes possible to form the GaInNAs film with a lattice constant that matches the lattice constant of GaAs. The GaInNAs film thus added with N has a reduced bandgap due to a large negative bowing of the bandgap-composition relationship observed in a GaAs-GaN system. Thus, it is expected that a double-heterostructure laser diode having an oscillation wavelength in the 1.3 or 1.5 .mu.m and simultaneously a large band discontinuity .DELTA.Ec necessary for carrier accumulation, may be obtained by using GaInNAs for the active layer. As the GaInNAs film can have a composition that establishes a lattice matching with GaAs, it is possible to use an AlGaAs cladding in combination with the active layer of GaInNAs. However, it should be noted that the GaInNAs mixed crystal system includes a large miscibility gap therein and the quality of the GaInNAs crystal thus obtained tends to be deteriorated when the N content and hence the laser oscillation wavelength are increased.
Further, there is a proposal to use an active layer of GaInTlP in a laser diode constructed on an InP substrate (Asahi, H., et al., Jpn. J. Appl. Phys. vol.35, pp.L876-L879, Part 2, No.7B, Jul. 15, 1996). It should be noted that TlP is a semi-metal having a negative bandgap and a lattice constant of about 0.6 nm, wherein this value of the lattice constant is about 3% larger than InP. Further, the bandgap of TlP widens with increasing temperature, contrary to a semiconductor material such as InP, in which the bandgap narrows with increasing temperature. Thus, by mixing InP and TlP with a suitable ratio, it is expected that a mixed crystal composition of InTIP is obtained in which the bandgap does not change substantially with temperature. Further, the active layer thus containing Tl may further contain Ga. In this case, the active layer has a composition of GaInTlP and establishes a lattice matching with an InP substrate.
While GaInTlP noted above is indeed a promising material for increasing the laser oscillation wavelength, the laser diode that uses GaInTlP for the active layer still suffers from the problem of poor carrier accumulation in the active layer and hence poor temperature characteristic, due to the use of an InP cladding layer. It should be noted that a cladding layer, having a substantial thickness, has to satisfy the requirement of lattice matching with the substrate, which in this case is InP. As long as InP is used for the cladding layer, the band discontinuity .DELTA.Ec between the GaInTlP active layer and the InP cladding layer is relatively limited.